The present invention relates to a motor vehicle headlamp employing a discharge bulb as its light source.
In the motor vehicle lamp industry, recently development efforts have been directed to a discharge lamp having an excellent light emission efficiency and color rendering characteristics, as well as a long service life. To achieve this, extensive research has been carried out on the use of a discharge lamp for the light source of a motor vehicle headlamp. However, the use of a discharge bulb suffers from a problem in that, in order to start the discharge bulb, it is necessary to provide a lighting circuit for applying high voltage across the two confronting electrodes of the discharge bulb. Moreover, the discharge lamp or lighting tube produces electromagnetic noise when activated, which can adversely affect nearby electrical circuits.
More particularly, in the operation of a discharge bulb, a metal halide bulb, for instance, typically produces electromagnetic waves in a frequency band 30 to 400 MHz. On the other hand, in order to start the discharge bulb, it is necessary to provide a lighting circuit for applying high voltage between the pair of electrodes employed in the discharge bulb. When the lighting tube in the lighting circuit is activated, electromagnetic waves are generated. Because of those electromagnetic waves, radio interference takes place. For instance, interference may be caused with the car radio or an on-board computer.